Removal of liquid hydrocarbons from empty compartments of ocean-going tankers



1952 R. L. STACK REMOVAL OF LIQUID HYDROCARBONS FROM EMPT COMPARTMENTS OF OCEAN-GOING TANKERS Filed March 27, 1958 INVENTOR. fioberf L. Stack A TTOR/VEY United States Patent ()fifice 3,067,069 Patented Dec. 4, 1962 3,067,069 REMOVAL OF LIQUID HYDROCARBONS FROM EMPTY COMPARTMENTS F OCEAN-GOING TANKERS Robert L. Stack, La Marque, Tex., assignor, by mesne assignments, to Standard Oil Company, Chicago, Ill., a corporation of Indiana Filed Mar. 27, 1958, Ser. No. 724,276 1 Claim. (Cl. 134-22) This is a continuation-in-part of my application Serial Number 500,277, filed April 8, 1955, now abandoned.

This invention relates to decreasing the corrosion in internal compartments of ocean-going tankers in liquid hydrocarbon service.

Tankers in liquid hydrocarbon Service normally operate either as clean vessels or as dirty vessels. Clean cargo includes gasoline, diesel fuel, jet fuel, and distillate fuel. Frequently a tanker carries one type of clean cargo in one voyage and must carry another type of clean cargo on the next voyage. Specifications on color, flash point, end point, etc. make it utterly impractical to intermingle in even a slight degree these deffirent liquid hydrocarbons. Therefore, it is necessary to remove substantially all of the liquid hydrocarbon remaining in the compartment of the tanker prior to introducing a difierent type of liquid hydrocarbon into that compartment. This residual hydrocarbon in the compartment consists of: small pools on the floor of the compartment due to unevenness of the steel surface, liquid caught in cracks and crevices; and as an adhering film on the vertical sides and roof of the compartment. l

Even when changing of cargoes is not involved, safety requires the freeing of empty compartments of hydrocarbon vapors. In a T2 tanker, an empty compartment will contain in light oil service, 45 barrels (42 gal.) of liquid residual hydrocarbon and vapors equal to about 40 barrels of liquid hydrocarbon. In the case of heavy oils such as crude and bunker fuel oils, blowing air through the empty compartment will usually reduce the vapor content below the hazard level, even though the bulk of the liquid residual oils remains on the metal surfaces. However, this simple procedure is not good enough with clean cargo to eliminate the vapor hazard.

It is customary to remove the residual hydrocarbon remaining in the compartment after the compartment has been emptied by thoroughly washing the entire compartment with water. Several techniques are used commercially wherein high pressure jets of water or aqueous cleaning fluid, at high temperatures, are driven against the metal surfaces present in the compartment. These surfaces include the walls of the compartment itself, the girders and other structural members which form part of the ships structure and are present in the compartment.

Owing to harbor pollution laws and atmospheric pollution laws, the non-ballast compartments cannot be freed of residual liquid hydrocarbons while the tanker is in harbor. Therefore, it is customary to use sea-water to remove residual liquid hydrocarbons from the non-ballast compartments; the cleaning is carried out after the tanker is at sea. The most common cleaning procedure is known as the Butterworth tank cleaning procedure, wherein sea water at about 200 F. and some hundred p.s.i. is sprayed against all the surfaces of the compartment. This procedure is described in detail in US. 2,550,997, column 1. The non-ballast compartments are those compartments which remain empty on the return voyage after discharge of the cargo at the delivery terminal. Certain of the compartments are filled with seaknown as ballast compartments.

An important drawback of the Butterworth tank cleaning procedure is the time needed to eliminate the presence of hydrocarbon vapors from the non-ballast compartments of the tanker. It is normal for 3 to 4 days to elapse from the time the cleaning of the non-ballast compartments has begun until the tanker is gas-free. During this period of time, there is considerable danger of fire, owing to the presence of hydrocarbon vapors. The rate of corrosion of the bulkheads of tankers in ordinary clean oil service, where frequent cleanings of the compartments are necessary, is so great that approximately every 10 years it has been necessary to completely replace the internal bulkheads of the vessel. At present construction costs, the rebuilding of a T2 tanker represents about $1,000,000.

A particular object of the invention is a. rapid method of removing liquid hydrocarbons from the empty compartments of ocean-going tankers, which method results in reduced corrosion rate of the steel surfaces present in the compartment and elimination of vapor hazard. Other objects will become apparent in the course of the detailed description.

In the method of the invention, fresh-water is sprayed onto the floor of the empty compartment; the mixture of water and oil formed in the compartment is removed during the spraying operation or imediately after the spraying is ended. The spraying is best controlled as a gentle shower-type and kept away from the sides of the compartment. The bottom washing operation may be likened to a rinsing procedure rather than the jet-action of the nozzles used in the Butterworth tank cleaning procedure. The amount of fresh-water used is dependent on the type of oil present and somewhat on the temperature of the water. Air is forced into the rinsed compartment to remove not only the hydrocarbon vapors already present but also to evaporate enough of the liquid hydrocarbon adhering to the sides and roof of the compartment to eliminate vapor hazard. When the compartment is gas-free, i.e., freed of hydrocarbon vapor, the vents are closed and the compartment then remains in a salt-free atmosphere condition.

Water is not used on the sides of the compartment because, it has been found, corrosion of the surface from which the liquid hydrocarbon has been removed by airblowing is far less than the surface where water-washing has been used. The combination of washing the floor to remove pools of liquid hydrocarbon and evaporation of the adhering film of liquid hydrocarbon from the sides of the compartment, removes enough of the residual hydrocarbon to permit transport of another type of clean cargo without adverse effect of physical characteristics thereof. For example, kerosene can follow gasoline without having its flash-point ruined or gasoline can follow kerosene without having its end-point ruined.

The fresh water utilized in the invention may be natural water such as collected rain water, water from flowing streams, lakes and ponds, i.e. Water containing an amount of natural mineral salts not suflicient to destroy potability.

The fresh water spray may be at ordinary atmospheric temperature or at somewhat higher temperature. In general, it is preferred to operate at temperatures attained by the water in the fresh water storage compartment of the tanker. By atmospheric temperature, it is to be understood to include those temperatures attained by the water while it is present in the compartment of the tanker, which temperature will vary -with the time of the year and the point on the earths surface at which the tanker is operating.

A T2 tanker in clean oil service was used as a test vessel in comparing the time requirements for gas-freeing a compartment containing residual hydrocarbon oil by the Butterworth tank cleaning procedure and by the method of this invention. The compartment was cleaned by the Butterworth tank cleaning procedure using seawater at a temperature of 170 F.; 7.5 hours were needed to remove all the residual oil and obtain a gas-free atmosphere in the compartment. A compartment of the same configuration was treated according to this method showering the bottom of the compartment with water at about 80 F.; the water-oil mixture was pumped out through the oil discharge lines of the tanker; compressed air was forced into the compartment after the hatches had been opened and the compartment was gas-free in 30 minutes. Thus, this method freed the compartment of residual clean oil in about one-fifteenth the time requirement of the Butterworth tank cleaning procedure. To clean all the non-ballast compartments of this tanker, 240 tons of purchased fresh water was required. This was carried in one of the compartments until the tanker was on the open sea where the cleaning was carried out.

By the use of the method of the instant invention, the corrosion rate of the steel surfaces of the non-ballast compartments is reduced from about 50% to about 30% of the rate incurred when utilizing sea water and the Butterworth tank cleaning procedure. Put in another way, the corrosion rate of the washed steel surfaces is only from about one-third to about one-half of the rate occurring when sea water and the Butterworth tank procedure is used to remove residual clean oil.

In the FIGURE there is shown a view of a tanker 11 and an oil compartment 12 being cleaned by the procedure of the invention. Fresh wash water from a source not shown is pumped into compartment 12 by way of hinged conduit 13, which is provided with a shower type spray head 14. Conduit 13 and head 14 permit Washing the bottom 16 of compartment 12 with a gentle spray of fresh water. The water and oil collecting on bottom 16 is passed out of compartment 12 by way of drain conduit 17 by means of a pump not shown. Air from compressor 18 is passed by way of line 19 into compartment 12. In this embodiment, conduit 13 and air line 19 are passed by way of manhole 21 into compartment 12. Exhaust air and hydrocarbon vapors exit from compartment 12 by way of manhole 22, and are discharged into the atmosphere by way of ventilator 23, which is a temporary structure adapted to be positioned over manhole 22. The spray of fresh wash water is directed over the entire bottom of the tank until the oil accumulated thereon has been washed oif and both the wash water and oil removed by way of conduit 17. Air is introduced by way of line 19 until the compartment 12 has been freed of hydrocarbon vapors to the level where no explosion hazard exists. At this time, the washing apparatus 1314, air line 19, and ventilator 23 are removed and the manholes 21 and 22 are then closed to maintain a salt-free atmosphere in oil compartment 12.

Thus having described the invention, what is claimed A method of decreasing corrosion and of eliminating the vapor hazard in empty non-ballast compartments of ocean-going tankers which carry clean cargo, which method comprises washing with potable fresh water at about atmospheric temperature, said wash water being applied essentially only to the floor of said compartment in a gentle-shower type manner whereby liquid hydrocarbon present on said floor is removed therefrom; substantially continuously withdrawing a water-hydrocarbon mixture produced by said washing action from said compartment during said washing operation; terminating said Washing and then blowing air into said compartment to remove hydrocarbon vapors therefrom, said air blowing being continued until essentially all of the hydrocarbon present on the walls and roof of said compartment has been vaporized therefrom and removed from said compartment and said compartment is substantially free of vapor hazard; and closing off said cleansed compartment in order to maintain a substantially salt-free atmosphere therein.

References Cited in the file of this patent UNITED STATES PATENTS 1,978,015 Erdman Oct. 23, 1934 2,443,721 Butcher June 22, 1948 2,497,946 Lester Feb. 21, 1950 2,550,997 Harden et al. May 1, 1951 2,661,241 Veneziano Dec. 1, 1953 FOREIGN PATENTS 1,092,149 France Nov. 3, 1954 

